Discharge lamp control device controlling lighting of a discharge lamp, and projector using the same

ABSTRACT

A discharge lamp control device includes: a discharge lamp driver section which drives a lamp; and a discharge lamp drive control section which performs constant current control so that a discharge lamp current that flows between electrodes of the lamp becomes constant when a discharge lamp voltage applied between the electrodes of the lamp is lower than a first voltage, and performs constant power control so that an amount of power supplied to the lamp becomes constant when the discharge lamp voltage is equal to or higher than the first voltage after the lamp has been lighted. The discharge lamp drive control section includes a discharge lamp drive frequency control section which causes the drive frequency when the discharge lamp voltage is lower than a second voltage which is equal to or lower than the first voltage to be lower than the drive frequency when the discharge lamp voltage is higher than the second voltage.

Japanese Patent Application No. 2007-32057, filed on Feb. 13, 2007, ishereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a discharge lamp control device and aprojector.

A discharge lamp such as a high-pressure mercury lamp or a metal halidelamp is used as a light source of a projector. When lighting a dischargelamp, a high voltage of several tens of kilovolts is normally generatedbetween electrodes of the discharge lamp in order to cause a dielectricbreakdown between the electrodes of the discharge lamp to form adischarge path. The voltage (discharge lamp voltage) between theelectrodes of the discharge lamp rapidly decreases immediately after thedischarge lamp has been lighted. In order to heat the electrodes of thedischarge lamp so that the discharge lamp voltage increases to a valuenear the rated voltage value, constant current control is performed sothat a discharge lamp current which flows between the electrodes of thedischarge lamp becomes constant when the discharge lamp voltage is lowerthan a predetermined voltage value. When the discharge lamp voltage hasbecome equal to or higher than the predetermined voltage value, constantpower control is performed so that the amount of power supplied to thedischarge lamp becomes constant. According to a related-art method, asshown in FIG. 7A, a discharge lamp current near the maximum designcurrent value of the discharge lamp is caused to flow during constantcurrent control in order to reduce the transition time from constantcurrent control to constant power control. In a discharge lamp lightingdevice disclosed in JP-A-9-82480, for example, the discharge lampcurrent during constant current control is set to be higher than thedischarge lamp current during constant power control.

However, the electrodes of the discharge lamp may be melted due to thehigh discharge lamp current during constant current control. Thedischarge lamp current during constant current control may be limited toprevent such a problem. However, since the discharge lamp is turned offif the discharge lamp current is limited to a large extent, thedischarge lamp current cannot be reduced to a large extent.

In order to extend the life of the discharge lamp, the discharge lampmay be driven by applying an alternating current so that a currentevenly flows between the electrodes of the discharge lamp. According toa related-art method, the drive frequency is set at a constantfrequency, as shown in FIG. 7B. However, the electrodes of the dischargelamp are easily damaged due to a large change in current when thepolarity of the discharge lamp current changes. Therefore, theelectrodes of the discharge lamp deteriorate when the drive frequency istoo high, whereby the life of the discharge lamp decreases.

SUMMARY

According to a first aspect of the invention, there is provided adischarge lamp control device which controls lighting of a dischargelamp, the discharge lamp control device comprising:

a discharge lamp driver section which drives the discharge lamp byoutputting an alternating current having a given drive frequency to thedischarge lamp; and

a discharge lamp drive control section which performs constant currentcontrol so that a discharge lamp current that flows between electrodesof the discharge lamp becomes constant when a discharge lamp voltageapplied between the electrodes of the discharge lamp is lower than afirst voltage, and performs constant power control so that an amount ofpower supplied to the discharge lamp becomes constant when the dischargelamp voltage is equal to or higher than the first voltage after thedischarge lamp has been lighted,

the discharge lamp drive control section including a discharge lampdrive frequency control section which causes the drive frequency whenthe discharge lamp voltage is lower than a second voltage which is equalto or lower than the first voltage to differ from the drive frequencywhen the discharge lamp voltage is higher than the second voltage.

According to a second aspect of the invention, there is provided aprojector comprising:

the above-described discharge lamp control device;

the discharge lamp;

an image signal input section which inputs an image signal; and

an image signal output section which outputs the image signal.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a functional block diagram showing a discharge lamp controldevice according to one embodiment of the invention.

FIG. 2 is a diagram illustrative of a configuration example of adown-converter (DC/DC conversion circuit).

FIG. 3 is a diagram illustrative of a configuration example of aninverter (DC/AC conversion circuit).

FIGS. 4A to 4F show graphs illustrative of a first example to a sixthexample of the control relationship between a discharge lamp voltage anda drive frequency.

FIGS. 5A to 5F show graphs illustrative of a first example to a sixthexample of the control relationship between a discharge lamp voltage anda discharge lamp current.

FIG. 6 is a diagram showing a configuration example of a projectoraccording to one embodiment of the invention.

FIG. 7A shows a graph of the control relationship between a dischargelamp voltage and a discharge lamp current according to a related-artmethod, and FIG. 7B shows a graph of the control relationship between adischarge lamp voltage and a drive frequency according to a related-artmethod.

DETAILED DESCRIPTION OF THE EMBODIMENT

The invention may provide a discharge lamp control device which preventselectrodes of a discharge lamp from being melted during constant currentcontrol after startup to extend the life of the discharge lamp, and aprojector.

(1) According to one embodiment of the invention, there is provided adischarge lamp control device which controls lighting of a dischargelamp, the discharge lamp control device comprising:

a discharge lamp driver section which drives the discharge lamp byoutputting an alternating current having a given drive frequency to thedischarge lamp; and

a discharge lamp drive control section which performs constant currentcontrol so that a discharge lamp current that flows between electrodesof the discharge lamp becomes constant when a discharge lamp voltageapplied between the electrodes of the discharge lamp is lower than afirst voltage, and performs constant power control so that an amount ofpower supplied to the discharge lamp becomes constant when the dischargelamp voltage is equal to or higher than the first voltage after thedischarge lamp has been lighted,

the discharge lamp drive control section including a discharge lampdrive frequency control section which causes the drive frequency whenthe discharge lamp voltage is lower than a second voltage which is equalto or lower than the first voltage to differ from the drive frequencywhen the discharge lamp voltage is higher than the second voltage.

The discharge lamp may be a discharge lamp driven using an alternatingcurrent. For example, the discharge lamp may be a high-pressure mercurylamp, a metal halide lamp, a xenon lamp, or the like used as a lightsource of a projector.

The alternating current having a given drive frequency is suppliedbetween the electrodes of the discharge lamp as the discharge lampcurrent. The alternating current having a given drive frequency may be asine wave or a rectangular wave, for example. When the alternatingcurrent is a rectangular wave, the rectangular wave contains anodd-order harmonic in addition to a fundamental harmonic. In this case,the drive frequency refers to the frequency of the fundamental harmonic.The drive frequency may be several tens to several hundreds of hertz, ormay be a higher frequency.

The first voltage value is a voltage value when a transition fromconstant current control to constant power control occurs after thedischarge lamp has been lighted. For example, the first voltage valuemay be a voltage value obtained by dividing the constant power valueduring constant power control by the constant current value duringconstant current control. It is preferable to set the first voltagevalue at a voltage value around the rated voltage in order to extend thelife of the discharge lamp.

The second voltage value equal to or lower than the first voltage valueis a discharge lamp voltage value within a range in which constantcurrent control is performed. It is preferable that the second voltagebe close to the first voltage value. The second voltage value may beequal to the first voltage value. In this case, the drive frequencyduring constant current control necessarily differs from the drivefrequency during constant power control.

When the drive frequency when the discharge lamp voltage is lower thanthe second voltage value is constant and the drive frequency when thedischarge lamp voltage is higher than the second voltage value is alsoconstant, these drive frequencies may differ from each other. The drivefrequency when the discharge lamp voltage is lower than the secondvoltage value may change and differ from the constant drive frequencywhen the discharge lamp voltage is higher than the second voltage value.

The discharge lamp control device according to the above embodiment mayinclude a discharge lamp voltage detection section which detects thedischarge lamp voltage and a discharge lamp current detection sectionwhich detects the discharge lamp current. The discharge lamp drivecontrol section may control the discharge lamp driver section based on avoltage value detected by the discharge lamp voltage detection sectionand a current value detected by the discharge lamp current detectionsection. In this case, the discharge lamp voltage detection section maydirectly or indirectly detect the voltage applied between the electrodesof the discharge lamp as the discharge lamp voltage. The discharge lampcurrent detection section may directly or indirectly detect the currentthat flows between the electrodes of the discharge lamp as the dischargelamp current. The discharge lamp current detection section may detectthe amplitude value of the discharge lamp current when the dischargelamp current is a rectangular wave, and may detect the amplitude valueor the root-mean-square value of the discharge lamp current when thedischarge lamp current is a sine wave.

According to the above embodiment, when the second voltage value isclose to the first voltage value, the drive frequency during almost theentire constant current control period is caused to differ from thedrive frequency during constant power control. Therefore, various typesof control are possible for various objectives. For example, the startuptime of the discharge lamp can be reduced by increasing the drivefrequency during constant current control. Moreover, since the number oftimes that the polarity of the discharge lamp current changes can bereduced by decreasing the drive frequency during constant currentcontrol, damage to the electrodes of the discharge lamp during constantcurrent control can be reduced.

(2) In this discharge lamp control device, the discharge lamp drivefrequency control section may cause the drive frequency when thedischarge lamp voltage is lower than the second voltage to be lower thanthe drive frequency when the discharge lamp voltage is higher than thesecond voltage.

As an example of the case where the drive frequency when the dischargelamp voltage is lower than the second voltage value is lower than thedrive frequency when the discharge lamp voltage is higher than thesecond voltage value, when the drive frequency when the discharge lampvoltage is lower than the second voltage value is constant and the drivefrequency when the discharge lamp voltage is higher than the secondvoltage value is also constant, the drive frequency when the dischargelamp voltage is lower than the second voltage value may be lower thanthe drive frequency when the discharge lamp voltage is higher than thesecond voltage value. The drive frequency when the discharge lampvoltage is lower than the second voltage value may change and be lowerthan the constant drive frequency when the discharge lamp voltage ishigher than the second voltage value.

The drive frequency when the discharge lamp voltage is lower than thesecond voltage value may be at least about half the drive frequency whenthe discharge lamp voltage is higher than the second voltage value. Forexample, the drive frequency when the discharge lamp voltage is lowerthan the second voltage value may be set at about 100 Hz to 150 Hz, andthe drive frequency when the discharge lamp voltage is higher than thesecond voltage value may be set at about 200 Hz to 300 Hz.

According to the above embodiment, when the second voltage value isclose to the first voltage value, the drive frequency during almost theentire constant current control period is lower than the drive frequencyduring constant power control. Therefore, the number of times that thepolarity of the discharge lamp current changes can be reduced duringconstant current control. As a result, the electrodes of the dischargelamp can be prevented from being melted even if a relatively largeamount of current is caused to flow between the electrodes of thedischarge lamp during constant current control. Therefore, the life ofthe discharge lamp can be extended.

(3) In this discharge lamp control device, when the discharge lampvoltage is within a predetermined range lower than the second voltage,the discharge lamp drive frequency control section may increase thedrive frequency according to an increase in the discharge lamp voltageso that the drive frequency almost successively changes when thedischarge lamp voltage is in the vicinity of the second voltage.

The term “predetermined range lower than the second voltage value” maybe a range between a voltage value equal to or higher than apredetermined voltage value lower than the second voltage value and avoltage value lower than the second voltage value, or may be a rangeincluding a plurality of predetermined ranges including the above range.

The statement “the drive frequency changes almost successively beforethe discharge lamp voltage reaches the second voltage value” used hereinexcludes a case where the drive frequency changes non-successively dueto a rapid change in drive frequency before the discharge lamp voltagereaches the second voltage value. It is preferable that the drivefrequency change gradually and successively before the discharge lampvoltage reaches the second voltage value.

The statement “increasing the drive frequency according to an increasein the discharge lamp voltage” used herein refers to a relationship inwhich the drive frequency necessarily increases as the discharge lampvoltage increases. For example, the drive frequency may linearly ornonlinearly increase with respect to the discharge lamp voltage.

According to the above embodiment, when the second voltage value isclose to the first voltage value, a situation in which the drivefrequency rapidly increases when a transition from constant currentcontrol to constant power control occurs can be prevented. This reducesthe load applied to the electrodes of the discharge lamp, whereby thelife of the discharge lamp can be extended.

(4) In this discharge lamp control device, the first voltage may beequal to the second voltage.

According to the above embodiment, since the drive frequency duringconstant current control is set to be lower than the drive frequencyduring constant power control, the number of times that the polarity ofthe discharge lamp current changes can be reduced during constantcurrent control. As a result, the electrodes of the discharge lamp canbe prevented from being melted even if a relatively large amount ofcurrent is caused to flow between the electrodes of the discharge lampduring constant current control. Therefore, the life of the dischargelamp can be extended.

(5) In this discharge lamp control device, the discharge lamp drivecontrol section may include a constant current control correctionsection which causes the discharge lamp current when the discharge lampvoltage is lower than a third voltage which is equal to or lower thanthe first voltage to be lower than the discharge lamp current when thedischarge lamp voltage is equal to the first voltage.

The third voltage value equal to or lower than the first voltage valueis a discharge lamp voltage value within a range in which constantcurrent control is performed. It is preferable that the third voltage beclose to the first voltage value. The third voltage value may be equalto the first voltage value.

The discharge lamp current when the discharge lamp voltage is equal tothe first voltage value is a current value calculated by dividing theconstant power supplied to the discharge lamp during constant powercontrol by the first voltage value.

According to the above embodiment, when the third voltage value is closeto the first voltage value, the discharge lamp current during almost theentire constant current control period is lower than the discharge lampcurrent immediately after transition to constant power control hasoccurred. Therefore, the load applied to the electrodes of the dischargelamp can be reduced during constant current control. This prevents theelectrodes of the discharge lamp from being melted, whereby the life ofthe discharge lamp can be extended.

(6) In this discharge lamp control device, when the discharge lampvoltage is within a predetermined range lower than the third voltage,the constant current control correction section may increase thedischarge lamp current according to an increase in the discharge lampvoltage so that the discharge lamp current almost successively changeswhen the discharge lamp voltage is in the vicinity of the third voltage.

The term “predetermined range lower than the third voltage value” may bea range between a voltage value equal to or higher than a predeterminedvoltage value lower than the third voltage value and a voltage valuelower than the third voltage value, or may be a range including aplurality of predetermined ranges including the above range.

The statement “the discharge lamp current changes almost successivelybefore the discharge lamp voltage reaches the third voltage value” usedherein excludes a case where the discharge lamp current changesnon-successively due to a rapid change in discharge lamp current beforethe discharge lamp voltage reaches the third voltage value. It ispreferable that the discharge lamp current change gradually andsuccessively before the discharge lamp voltage reaches the third voltagevalue.

The statement “increasing the discharge lamp current according to anincrease in the discharge lamp voltage” used herein refers to arelationship in which the discharge lamp current necessarily increasesas the discharge lamp voltage increases. For example, the discharge lampcurrent may linearly or nonlinearly increase with respect to thedischarge lamp voltage.

According to the above embodiment, when the third voltage value is closeto the first voltage value, a situation in which the discharge lampcurrent rapidly increases when a transition from constant currentcontrol to constant power control occurs can be prevented. This reducesthe load applied to the electrodes of the discharge lamp, whereby thelife of the discharge lamp can be extended.

(7) In this discharge lamp control device, the first voltage may beequal to the third voltage.

According to the above embodiment, since the discharge lamp currentduring the constant current control period is set to be lower than thedischarge lamp current immediately after transition to constant powercontrol has occurred, the load applied to the electrodes of thedischarge lamp can be reduced during constant current control. Thisprevents the electrodes of the discharge lamp from being melted, wherebythe life of the discharge lamp can be extended.

(8) In this discharge lamp control device, the discharge lamp may be alight source of a projector.

According to the above embodiment, a discharge lamp control device whichextends the life of a projector discharge lamp can be provided.

(9) According to one embodiment of the invention, there is provided aprojector comprising:

the above-described discharge lamp control device;

the discharge lamp;

an image signal input section which inputs an image signal; and

an image signal output section which outputs the image signal.

According to the above embodiment, a projector which does not requirereplacement of a discharge lamp for a long period of time can beprovided.

Some embodiments of the invention will be described in detail below,with reference to the drawings. Note that the embodiments describedbelow do not in any way limit the scope of the invention laid out in theclaims herein. In addition, not all of the elements of the embodimentsdescribed below should be taken as essential requirements of theinvention.

1. Discharge Lamp Control Device

FIG. 1 is a functional block diagram showing a discharge lamp controldevice according to one embodiment of the invention. A discharge lampcontrol device 10 controls lighting of a lamp (discharge lamp) 20. Thedischarge lamp control device 10 includes a discharge lamp driversection 100. The discharge lamp driver section 100 outputs analternating current (discharge lamp current) 142 having a given drivefrequency to drive the lamp (discharge lamp) 20. The discharge lampdriver section 100 includes an input filter 110, a down-converter (DC/DCconversion circuit) 120, an inverter (DC/AC conversion circuit) 130, anda high-voltage generation circuit (igniter) 140. The input filter 110removes a high-frequency noise component contained in a direct-currentinput signal 12. The down-converter 120 decreases the voltage of adirect-current signal 112 (e.g., direct-current signal at 380 V) outputfrom the input filter 110 to convert the direct-current signal 112 intoa direct-current signal 122 (e.g., direct-current signal at 50 V to 130V). The inverter 130 converts the direct-current signal 122 output fromthe down-converter 120 into an alternating-current signal 132(alternating-current rectangular wave at several tens to severalhundreds of hertz). The high-voltage generation circuit 140 generates ahigh voltage of several tens of kilovolts between the electrodes of thelamp 20 in order to cause a dielectric breakdown between the electrodesof the lamp 20 to form a discharge path when lighting the lamp 20. Theoperation of the high-voltage generation circuit 140 is stopped afterthe lamp 20 has been lighted, and the alternating-current signal 132 issupplied between the electrodes of the lamp 20 as the discharge lampcurrent 142. Note that the input filter 110, the down-converter 120, theinverter 130, and the high-voltage generation circuit 140 are notessential elements of the discharge lamp driver section 100. Thedischarge lamp driver section 100 may have a configuration other thanthe above-described configuration.

The discharge lamp control device 10 may include a current detectioncircuit 200. The current detection circuit 200 functions as a dischargelamp current detection section which detects the discharge lamp currentwhich flows between the electrodes of the lamp (discharge lamp) 20. Forexample, the current detection circuit 200 detects the direct current122 output from the down-converter 120, and supplies a detected currentvalue 202 to a system controller 410. The current detection circuit 200may directly or indirectly detect the discharge lamp current which flowsbetween the electrodes of the lamp 20.

The discharge lamp control device 10 may include a voltage detectioncircuit 300. The voltage detection circuit 300 functions as a dischargelamp voltage detection section which detects the discharge lamp voltageapplied between the electrodes of the lamp (discharge lamp) 20. Forexample, the voltage detection circuit 300 detects thealternating-current voltage 132 output from the inverter 130, andsupplies a detected voltage value 302 to the system controller 410. Thevoltage detection circuit 300 may directly or indirectly detect thedischarge lamp voltage applied between the electrodes of the lamp 20.

The discharge lamp control device 10 includes a discharge lamp drivecontrol section 400. After the lamp (discharge lamp) 20 has beenlighted, the discharge lamp drive control section 400 performs constantcurrent control so that the discharge lamp current 142 becomes constantwhen the discharge lamp voltage is lower than a first voltage value, andperforms constant power control so that the amount of power supplied tothe lamp (discharge lamp) 20 becomes constant when the discharge lampvoltage is equal to or higher than the first voltage value. Thedischarge lamp drive control section 400 includes the system controller410, a power control circuit 420, a controller 430, and a frequencysetting circuit 440.

The system controller 410 issues instructions to the power controlcircuit 420, the controller 430, and the frequency setting circuit 440based on a control signal 30 to control the discharge lamp current 142output from the discharge lamp driver section 100. For example, thesystem controller 410 instructs the high-voltage generation circuit 140to generate a high voltage between the electrodes of the lamp 20 basedon the control signal 30 in order to cause a dielectric breakdownbetween the electrodes of the lamp 20 to form a discharge pathimmediately after power has been supplied.

The system controller 410 calculates the present discharge lamp voltagefrom the voltage value 302 detected by the voltage detection circuit 300after the lamp 20 has been lighted. The system controller 410 performsconstant current control when the system controller 410 has determinedthat the discharge lamp voltage is lower than the first voltage value,and performs constant power control when the system controller 410 hasdetermined that the discharge lamp voltage is equal to or higher thanthe first voltage value. When the system controller 410 performsconstant current control, the system controller 410 calculates thepresent discharge lamp current from the current value 202 detected bythe current detection circuit 200, and instructs the controller 430 togenerate a control signal 432 for keeping the discharge lamp currentconstant to control the voltage or current of the direct-current signal122 output from the down-converter 120. Specifically, the systemcontroller 410 increases the voltage or current of the direct-currentsignal 122 when the present discharge lamp current is lower than aconstant current value, and decreases the voltage or current of thedirect-current signal 122 when the present discharge lamp current ishigher than the constant current value. When the system controller 410performs constant power control, the system controller 410 calculatesthe amount of power (i.e., the product of the discharge lamp current andthe discharge lamp voltage) currently supplied to the lamp 20, andsupplies information relating to the present power value to the powercontrol circuit 420. The power control circuit 420 instructs thecontroller 430 to generate the control signal 432 for keeping the amountof power supplied to the lamp 20 constant to control the voltage orcurrent of the direct-current signal 122 output from the down-converter120. Specifically, the power control circuit 420 increases the voltageor current of the direct-current signal 122 when the amount of powercurrently supplied to the lamp 20 is lower than the constant powervalue, and decreases the voltage or current of the direct-current signal122 when the amount of power currently supplied to the lamp 20 is higherthan the constant power value.

The system controller 410 and the frequency setting circuit 440 functionas a discharge lamp drive frequency control section which causes thedrive frequency when the discharge lamp voltage is lower than a secondvoltage value equal to or lower than the first voltage value to differfrom the drive frequency when the discharge lamp voltage is higher thanthe second voltage value. The system controller 410 and the frequencysetting circuit 440 may function as a discharge lamp drive frequencycontrol section which causes the drive frequency when the discharge lampvoltage is lower than the second voltage value to be lower than thedrive frequency when the discharge lamp voltage is higher than thesecond voltage value. For example, when the first voltage value is equalto the second voltage value, the system controller 410 may supply apredetermined setting value to the frequency setting circuit 440 so thatthe frequency of the alternating-current signal 132 (drive frequency ofthe discharge lamp current 142) output from the inverter 130 duringconstant current control is lower than the frequency of thealternating-current signal 132 during constant power control, and thefrequency setting circuit 440 may generate a control signal 442 based onthe setting value to control the inverter 130. The system controller 410and the frequency setting circuit 440 may function as a discharge lampdrive frequency control section which increases the drive frequencyaccording to an increase in the discharge lamp voltage so that the drivefrequency changes almost successively before the discharge lamp voltagereaches the second voltage value when the discharge lamp voltage isequal to a voltage value within a predetermined range lower than thesecond voltage value.

The system controller 410 and the controller 430 may function as aconstant current control correction section which makes a correction sothat the discharge lamp current when the discharge lamp voltage is lowerthan a third voltage value equal to or lower than the first voltagevalue is lower than the discharge lamp current when the discharge lampvoltage is equal to the first voltage value. For example, when the firstvoltage value is equal to the third voltage value, the system controller410 may instruct the controller 430 to generate the control signal 432which causes the discharge lamp current during constant current controlto be lower than the discharge lamp current when the discharge lampvoltage is equal to the first voltage value to correct the voltage orcurrent of the direct-current signal 122 output from the down-converter120. The system controller 410 and the controller 430 may function as aconstant current control correction section which increases thedischarge lamp current according to an increase in the discharge lampvoltage so that the discharge lamp current changes almost successivelybefore the discharge lamp voltage reaches the third voltage value whenthe discharge lamp voltage is equal to a voltage value within apredetermined range lower than the third voltage value.

Note that the system controller 410, the power control circuit 420, thecontroller 430, and the frequency setting circuit 440 are not essentialelements of the discharge lamp drive control section 400. The dischargelamp drive control section 400 may have a configuration other than theabove-described configuration. The discharge lamp drive control section400 may be implemented by hardware (i.e., dedicated circuit), or may beimplemented by software (i.e., control program which can be executed bya general-purpose CPU), or may be implemented by hardware and software.

FIG. 2 is a diagram illustrative of a configuration example of thedown-converter (DC/DC conversion circuit). The down-converter (DC/DCconversion circuit) 120 is configured as a step-down chopper circuitincluding an NPN-type transistor T1, a diode D1, a coil L1, and acapacitor C1. A collector terminal of the transistor T1 is connected toan input terminal I1. An emitter terminal of the transistor T1 isconnected to a cathode terminal of the diode D1 and one end of the coilL1. A base terminal of the transistor T1 is connected to an inputterminal I3. One end of the capacitor C1 is connected to the other endof the coil L1 and an output terminal O1. The other end of the capacitorC1 is connected to an anode terminal of the diode D1, an input terminalI2, and an output terminal O2. The output terminals O1 and O2 areconnected to the inverter 130 (see FIG. 1). The direct-current signal122 (see FIG. 1) is output from the output terminal O1. Thedirect-current signal 112 (see FIG. 1) output from the input filter 110(see FIG. 1) is supplied to the input terminal I1. A constant potential(e.g., ground potential) is supplied to the input terminal I2. Aconstant direct-current voltage (e.g., 380 V) is applied between theinput terminals I1 and I2. The control signal 432 (see FIG. 1) whichON/OFF-controls the transistor T1 is supplied to the input terminal I3.

When the transistor T1 is turned ON, a current flows through the coil L1so that energy is stored in the coil L1. When the transistor T1 isturned OFF, the energy stored in the coil L1 is discharged through apath which passes through the capacitor C1 and the diode D1. As aresult, the voltage applied between the input terminals I1 and I2 isdecreased so that a direct-current voltage (e.g., 50 V to 130 V)proportional to the ratio of the period of time in which the transistorT1 is turned ON is generated between the output terminals O1 and O2.

For example, the system controller 410 shown in FIG. 1 causes apredetermined direct-current voltage to be generated between the outputterminals O1 and 02 by adjusting the duty of the control signal 432generated by the controller 430 to ON/OFF-control the transistor T1.Specifically, the system controller 410 causes a predetermineddirect-current voltage to be generated between the output terminals O1and O2 during constant current control so that the discharge lampcurrent is equal to a desired constant current value, and adjusts theduty of the control signal 432 during constant power control so that apredetermined direct-current voltage is generated between the outputterminals O1 and O2 so that the amount of power (i.e., the product ofthe discharge lamp current and the discharge lamp voltage) supplied tothe discharge lamp is equal to a desired constant power value.

A resistor may be inserted between the anode terminal of the diodes D1and the input terminal I2, and the current detection circuit 200 (seeFIG. 1) may detect a current which flows through the resistor. Tworesistors may be connected in series between the output terminal O1 andground, and the voltage detection circuit 300 (see FIG. 1) may detect avoltage divided at the resistance ratio of the resistors.

FIG. 3 is a diagram illustrative of a configuration example of theinverter (DC/AC conversion circuit). The inverter (DC/AC conversioncircuit) 130 is configured as a full-bridge inverter circuit includingfour NPN-type transistors T2 to T5, a coil L2, and a capacitor C2. Acollector terminal of the transistor T2 is connected to an inputterminal 14 and a collector terminal of the transistor T4. An emitterterminal of the transistor T2 is connected to a collector terminal ofthe transistor T3 and one end of the coil L2. A base terminal of thetransistor T2 is connected to an input terminal 16. An emitter terminalof the transistor T5 is connected to an input terminal 15 and an emitterterminal of the transistor T3. A collector terminal of the transistor T5is connected to an emitter terminal of the transistor T4, one end of thecoil L2, and an output terminal O4. A base terminal of the transistor T5is connected to an input terminal I9. A base terminal of the transistorT3 is connected to an input terminal 17. A base terminal of thetransistor T4 is connected to an input terminal I8. The other end of thecapacitor C2 is connected to the other end of the coil L2 and an outputterminal O3. The output terminals O3 and O4 are respectively connectedto the electrodes of the lamp 20 (see FIG. 1). The alternating-currentsignal 132 (see FIG. 1) is output through the output terminals O3 andO4. The high-voltage generation circuit 140 (see FIG. 1) is connectedbetween the output terminals O3 and O4. The output terminals O1 and O2(see FIG. 2) of the down-converter 120 are respectively connected to theinput terminals I4 and I5. The direct-current signal 122 (FIG. 1 and seeFIG. 2) output from the down-converter 120 is supplied to the inputterminal I4. A constant potential (e.g., ground potential) is suppliedto the input terminal I5. A constant direct-current voltage (e.g., 50 Vto 130 V) is applied between the input terminals I4 and I5. The controlsignals 442 (see FIG. 1) which respectively ON/OFF-control thetransistors T2 and T5 are supplied to the input terminals I6 and I9.Control signals 442′ (inverted signal of the control signal 442) whichrespectively ON/OFF-control the transistors T3 and T4 are supplied tothe input terminals I7 and I8. Specifically, the transistors T3 and T4are turned OFF when the transistors T2 and T5 are turned ON, and thetransistors T3 and T4 are turned ON when the transistors T2 and T5 areturned OFF. Therefore, the alternating-current discharge lamp current142 (see FIG. 1) of which the polarity changes cyclically is suppliedbetween the electrodes of the lamp 20 (see FIG. 1) connected to theoutput terminals O3 and O4 by causing the transistors T2 and T5 and thetransistors T3 and T4 to be exclusively turned ON/OFF in a predeterminedcycle. Since the ON/OFF frequency of the transistors T2 to T5 serves asthe drive frequency, the system controller 410 shown in FIG. 1 adjuststhe frequency of the control signal 442 (e.g., rectangular-wave signal)generated by the frequency setting circuit 440 so that the dischargelamp current 142 having a predetermined drive frequency is suppliedbetween the electrodes of the lamp 20 through the output terminals O3and O4.

FIGS. 4A to 4F show graphs illustrative of the control relationshipbetween the discharge lamp voltage and the drive frequency. In FIGS. 4Ato 4F, the horizontal axis indicates the discharge lamp voltage, and thevertical axis indicates the drive frequency. Constant current control isperformed when the discharge lamp voltage is lower than a voltage valueV1 (first voltage value), and constant power control is performed whenthe discharge lamp voltage is equal to or higher than the voltage valueV1. In FIGS. 4A to 4F, the drive frequency when the discharge lampvoltage is lower than a voltage value V2 (second voltage value) equal toor lower than the voltage value V1 is set to be lower than the drivefrequency when the discharge lamp voltage is higher than the voltagevalue V2.

FIG. 4A is a view illustrative of a first example of the controlrelationship between the discharge lamp voltage and the drive frequency.In FIG. 4A, the drive frequency when the discharge lamp voltage is lowerthan the voltage value V2 is set at a constant value F2. The drivefrequency when the discharge lamp voltage is higher than the voltagevalue V2 is set at a constant value F1.

FIG. 4B is a view illustrative of a second example of the controlrelationship between the discharge lamp voltage and the drive frequency.In FIG. 4B, the drive frequency when the discharge lamp voltage is lowerthan a voltage value V3 is set at the constant value F2. When thedischarge lamp voltage is set at a voltage value in the range betweenthe voltage values V3 and V2 (voltage value within a predetermined rangelower than the second voltage value), the drive frequency is increasedfrom the value F2 to the value F1 according to an increase in thedischarge lamp voltage so that the drive frequency successively changesto the value F1 before the discharge lamp voltage reaches the voltagevalue V2 (second voltage value). The drive frequency when the dischargelamp voltage is higher than the voltage value V2 is set at the constantvalue F1.

FIG. 4C is a view illustrative of a third example of the controlrelationship between the discharge lamp voltage and the drive frequency.In FIG. 4C, when the discharge lamp voltage is set at a voltage valuelower than the voltage value V2 (voltage value within a predeterminedrange lower than the second voltage value), the drive frequency isincreased from the value F2 to the value F1 according to an increase inthe discharge lamp voltage so that the drive frequency successivelychanges to the value F1 before the discharge lamp voltage reaches thevoltage value V2 (second voltage value). The drive frequency when thedischarge lamp voltage is higher than the voltage value V2 is set at theconstant value F1.

According to the control relationships shown in FIGS. 4A to 4C, thenumber of times that the polarity of the discharge lamp current changesduring constant current control can be reduced when the voltage value V1is relatively close to the voltage value V2. As a result, the electrodesof the discharge lamp can be prevented from being melted even if arelatively large amount of current is caused to flow between theelectrodes of the discharge lamp during constant current control,whereby the life of the discharge lamp can be extended. According to thecontrol relationship shown in FIG. 4B or 4C, a situation in which thefrequency of the discharge lamp current rapidly increases before atransition from constant current control to constant power controloccurs can be prevented so that the load applied to the electrodes ofthe discharge lamp can be reduced.

FIG. 4D is a view illustrative of a fourth example of the controlrelationship between the discharge lamp voltage and the drive frequency.FIG. 4D shows the case where the voltage value V1 (first voltage value)is equal to the voltage value V2 (second voltage value) in the controlrelationship shown in FIG. 4A. Specifically, the drive frequency whenthe discharge lamp voltage is lower than the voltage value V1 (=V2)(during constant current control) is set at the constant value F2. Thedrive frequency when the discharge lamp voltage is higher than thevoltage value V1 (during constant power control) is set at the constantvalue F1.

FIG. 4E is a view illustrative of a fifth example of the controlrelationship between the discharge lamp voltage and the drive frequency.FIG. 4E shows the case where the voltage value V1 (first voltage value)is equal to the voltage value V2 (second voltage value) in the controlrelationship shown in FIG. 4B. Specifically, the drive frequency whenthe discharge lamp voltage is lower than the voltage value V3 is set atthe constant value F2. When the discharge lamp voltage is set at avoltage value in the range between the voltage value V3 and the voltagevalue V1 (=V2) (voltage value within a predetermined range lower thanthe second voltage value), the drive frequency is increased from thevalue F2 to the value F1 according to an increase in the discharge lampvoltage so that the drive frequency successively changes to the value F1before the discharge lamp voltage reaches the voltage value V1 (=V2(second voltage value)). The drive frequency when the discharge lampvoltage is higher than the voltage value V1 (during constant powercontrol) is set at the constant value F1.

FIG. 4F is a view illustrative of a sixth example of the controlrelationship between the discharge lamp voltage and the drive frequency.FIG. 4F shows the case where the voltage value V1 (first voltage value)is equal to the voltage value V2 (second voltage value) in the controlrelationship shown in FIG. 4C. Specifically, when the discharge lampvoltage is set at a voltage value lower than the voltage value V1 (=V2)(voltage value within a predetermined range lower than the secondvoltage value), the drive frequency is increased from the value F2 tothe value F1 according to an increase in the discharge lamp voltage sothat the drive frequency successively changes to the value F1 before thedischarge lamp voltage reaches the voltage value V1 (=V2 (second voltagevalue)). The drive frequency when the discharge lamp voltage is higherthan the voltage value V1 (during constant power control) is set at theconstant value F1.

According to the control relationships shown in FIGS. 4D to 4F, thenumber of times that the polarity of the discharge lamp current changesduring constant current control can be reduced. As a result, theelectrodes of the discharge lamp can be prevented from being melted evenif a relatively large amount of current is caused to flow between theelectrodes of the discharge lamp during constant current control,whereby the life of the discharge lamp can be extended. According to thecontrol relationship shown in FIG. 4E or 4F, a situation in which thefrequency of the discharge lamp current rapidly increases when atransition from constant current control to constant power controloccurs can be prevented so that the load applied to the electrodes ofthe discharge lamp can be reduced.

In the control relationship shown in FIGS. 4B and 4E, the drivefrequency is nonlinearly increased from the value F2 to the value F1.Note that the drive frequency may be linearly increased from the valueF2 to the value F1. In the control relationship shown in FIGS. 4C and4F, the drive frequency is linearly increased from the value F2 to thevalue F1. Note that the drive frequency may be nonlinearly increasedfrom the value F2 to the value F1.

FIGS. 5A to 5F show graphs illustrative of the control relationshipbetween the discharge lamp voltage and the discharge lamp current. InFIGS. 5A to 5F, the horizontal axis indicates the discharge lampvoltage, and the vertical axis indicates the discharge lamp current.Constant current control is performed when the discharge lamp voltage islower than the voltage value V1 (first voltage value), and constantpower control is performed when the discharge lamp voltage is equal toor higher than the voltage value V1. In FIGS. 5A to 5F, a correction ismade so that the discharge lamp current when the discharge lamp voltageis lower than the voltage value V2 (third voltage value) equal to orlower than the voltage value V1 is lower than the discharge lamp currentwhen the discharge lamp voltage is equal to the voltage value V1 (firstvoltage value). In FIGS. 5A to 5F, when the discharge lamp voltage ishigher than the voltage value V1 (during constant power control), thedischarge lamp current is in inverse proportion to the discharge lampvoltage so that the amount of power supplied to the discharge lamp isconstant.

FIG. 5A is a view illustrative of a first example of the controlrelationship between the discharge lamp voltage and the discharge lampcurrent. In FIG. 5A, the discharge lamp current when the discharge lampvoltage is lower than the voltage value V2 is set at a constant valueI2. The discharge lamp current when the discharge lamp voltage is set ata voltage value between the voltage values V2 and V1 is set at aconstant value I1.

FIG. 5B is a view illustrative of a second example of the controlrelationship between the discharge lamp voltage and the discharge lampcurrent. In FIG. 5B, the discharge lamp current when the discharge lampvoltage is lower than the voltage value V3 is set at the constant valueI2. When the discharge lamp voltage is set at a voltage value betweenthe voltage values V3 and V2 (voltage value within a predetermined rangelower than the third voltage value), the discharge lamp current isincreased from the value I2 to the value I1 according to an increase inthe discharge lamp voltage so that the discharge lamp currentsuccessively changes to the value I1 before the discharge lamp voltagereaches the voltage value V2 (third voltage value). The discharge lampcurrent when the discharge lamp voltage is set at a voltage valuebetween the voltage values V2 and V1 is set at the constant value I1.

FIG. 5C is a view illustrative of a third example of the controlrelationship between the discharge lamp voltage and the discharge lampcurrent. In FIG. 5C, when the discharge lamp voltage is set at a voltagevalue lower than the voltage value V2 (voltage value within apredetermined range lower than the third voltage value), the dischargelamp current is increased from the value I2 to the value I1 according toan increase in the discharge lamp voltage so that the discharge lampcurrent successively changes to the value I1 before the discharge lampvoltage reaches the voltage value V2 (third voltage value). Thedischarge lamp current when the discharge lamp voltage is set at avoltage value between the voltage values V2 and V1 is set at theconstant value I1.

According to the control relationships shown in FIGS. 5A to 5C, thedischarge lamp current during constant current control can be reducedwhen the voltage value V1 is relatively close to the voltage value V2.As a result, the electrodes of the discharge lamp can be prevented frombeing melted during constant current control, whereby the life of thedischarge lamp can be extended. According to the control relationshipshown in FIG. 5B or 5C, a situation in which the discharge lamp currentrapidly increases before a transition from constant current control toconstant power control occurs can be prevented so that the load appliedto the electrodes of the discharge lamp can be reduced.

FIG. 5D is a view illustrative of a fourth example of the controlrelationship between the discharge lamp voltage and the discharge lampcurrent. FIG. 5D shows the case where the voltage value V1 (firstvoltage value) is equal to the voltage value V2 (third voltage value) inthe control relationship shown in FIG. 5A. Specifically, the dischargelamp current when the discharge lamp voltage is lower than the voltagevalue V1 (=V2) (during constant current control) is set at the constantvalue I2.

FIG. 5E is a view illustrative of a fifth example of the controlrelationship between the discharge lamp voltage and the discharge lampcurrent. FIG. 5E shows the case where the voltage value V1 (firstvoltage value) is equal to the voltage value V2 (third voltage value) inthe control relationship shown in FIG. 5B. Specifically, the dischargelamp current when the discharge lamp voltage is lower than the voltagevalue V3 is set at the constant value I2. When the discharge lampvoltage is set at a voltage value in the range between the voltage valueV3 and the voltage value V1 (=V2) (voltage value within a predeterminedrange lower than the third voltage value), the discharge lamp current isincreased from the value I2 to the value I1 according to an increase inthe discharge lamp voltage so that the discharge lamp currentsuccessively changes to the value I1 before the discharge lamp voltagereaches the voltage value V1 (=V2 (third voltage value)).

FIG. 5F is a view illustrative of a sixth example of the controlrelationship between the discharge lamp voltage and the discharge lampcurrent. FIG. 5F shows the case where the voltage value V1 (firstvoltage value) is equal to the voltage value V2 (third voltage value) inthe control relationship shown in FIG. 5C. Specifically, when thedischarge lamp voltage is set at a voltage value lower than the voltagevalue V1 (=V2) (voltage value within a predetermined range lower thanthe third voltage value), the discharge lamp current is increased fromthe value I2 to the value I1 according to an increase in the dischargelamp voltage so that the discharge lamp current successively changes tothe value I1 before the discharge lamp voltage reaches the voltage valueV1 (=V2 (third voltage value)).

According to the control relationships shown in FIGS. 5D to 5F, thedischarge lamp current can be reduced during constant current control.As a result, the electrodes of the discharge lamp can be prevented frombeing melted during constant current control, whereby the life of thedischarge lamp can be extended. According to the control relationshipshown in FIG. 5E or 5F, a situation in which the discharge lamp currentrapidly increases when a transition from constant current control toconstant power control occurs can be prevented so that the load appliedto the electrodes of the discharge lamp can be reduced.

In the control relationship shown in FIGS. 5B and 5E, the discharge lampcurrent is nonlinearly increased from the value I2 to the value I1. Notethat the discharge lamp current may be linearly increased from the valueI2 to the value I1. In the control relationship shown in FIGS. 5C and5F, the discharge lamp current is linearly increased from the value I12to the value I1. Note that the discharge lamp current may be nonlinearlyincreased from the value I2 to the value I1.

It is effective to combine the control relationship between thedischarge lamp voltage and the drive frequency described with referenceto FIGS. 4A to 4F and the control relationship between the dischargelamp voltage and the discharge lamp current described with reference toFIGS. 5A to 5F. For example, if the discharge lamp current is reduced tosuch an extent that the discharge lamp is not turned off while reducingthe drive frequency during constant current control, the effects ofpreventing the electrodes of the discharge lamp from being melted andextending the life of the discharge lamp can be improved as comparedwith the case of reducing only one of the drive frequency and thedischarge lamp current.

2. Projector

FIG. 6 shows a configuration example of a projector according to oneembodiment of the invention. A projector 500 includes an image signalconversion section 510, a power supply device 520, a discharge lampcontrol device 530, a lamp 540, a mirror group 550, liquid crystalpanels 560R, 560G, 560B, and an image processing device 570. The imagesignal conversion section 510 converts an externally input image signal502 (e.g., luminance-color difference signal or digital RGB signal) intoan analog RGB signal to generate image signals 512R, 512G, and 512B, andsupplies the image signals 512R, 512G, and 512B to the image processingdevice 570. The image processing device 570 processes the image signals512R, 512G, and 512B, and outputs drive signals 572R, 572G, and 572B fordriving the liquid crystal panels 560R, 560G, 560B, respectively.

The power supply device 520 converts an alternating-current voltagesupplied from an external alternating-current power supply 600 into aconstant direct-current voltage, and supplies the direct-current voltageto the image signal conversion section 510 and the image processingdevice 570 on the secondary-side of a transformer (not shown; includedin the power supply device 520) and to the discharge lamp control device530 on the primary-side instrument of the transformer. The dischargelamp control device 530 generates a high voltage between the electrodesof the lamp 540 to cause a dielectric breakdown between the electrodesso that a discharge path is formed, and then supplies a lamp current(discharge lamp current) for the lamp 540 to maintain discharge. A beamemitted from the lamp 540 is separated into R, G, and B beams throughtwo dichroic mirrors included in the mirror group 550. The beams arereflected by other mirrors to enter the liquid crystal panels 560R,560G, 560B. The liquid crystal panels 560R, 560G, 560B display imagesbased on the drive signals 572R, 572G, and 572B, respectively. The R, G,and B beams pass through the liquid crystal panels 560R, 560G, 560B andare synthesized by a prism, and the resulting image is displayed on ascreen 700.

The invention is not limited to the above-described embodiments, andvarious modifications can be made within the scope of the invention.

The invention includes various other configurations substantially thesame as the configurations described in the embodiments (in function,method and result, or in objective and result, for example). Theinvention also includes a configuration in which an unsubstantialportion in the described embodiments is replaced. The invention alsoincludes a configuration having the same effects as the configurationsdescribed in the embodiments, or a configuration able to achieve thesame objective. Further, the invention includes a configuration in whicha publicly known technique is added to the configurations in theembodiments.

Although only some embodiments of this invention have been described indetail above, those skilled in the art will readily appreciate that manymodifications are possible in the embodiments without materiallydeparting from the novel teachings and advantages of this invention.Accordingly, all such modifications are intended to be included withinthe scope of the invention.

1. A discharge lamp control device which controls lighting of adischarge lamp, the discharge lamp control device comprising: adischarge lamp driver section which drives the discharge lamp byoutputting an alternating current having a given drive frequency to thedischarge lamp; and a discharge lamp drive control section whichperforms constant current control so that a discharge lamp current thatflows between electrodes of the discharge lamp becomes constant when adischarge lamp voltage applied between the electrodes of the dischargelamp is lower than a first voltage, and performs constant power controlso that an amount of power supplied to the discharge lamp becomesconstant when the discharge lamp voltage is equal to or higher than thefirst voltage after the discharge lamp has been lighted, the dischargelamp drive control section including a discharge lamp drive frequencycontrol section which causes the drive frequency when the discharge lampvoltage is lower than a second voltage which is equal to or lower thanthe first voltage to differ from the drive frequency when the dischargelamp voltage is higher than the second voltage.
 2. The discharge lampcontrol device as defined in claim 1, the discharge lamp drive frequencycontrol section causing the drive frequency when the discharge lampvoltage is lower than the second voltage to be lower than the drivefrequency when the discharge lamp voltage is higher than the secondvoltage.
 3. The discharge lamp control device as defined in claim 2,when the discharge lamp voltage is within a predetermined range lowerthan the second voltage, the discharge lamp drive frequency controlsection increasing the drive frequency according to an increase in thedischarge lamp voltage so that the drive frequency almost successivelychanges when the discharge lamp voltage is in the vicinity of the secondvoltage.
 4. The discharge lamp control device as defined in claim 1, thefirst voltage being equal to the second voltage.
 5. The discharge lampcontrol device as defined in claim 2, the first voltage being equal tothe second voltage.
 6. The discharge lamp control device as defined inclaim 3, the first voltage being equal to the second voltage.
 7. Thedischarge lamp control device as defined in claim 1, the discharge lampdrive control section including a constant current control correctionsection which causes the discharge lamp current when the discharge lampvoltage is lower than a third voltage which is equal to or lower thanthe first voltage to be lower than the discharge lamp current when thedischarge lamp voltage is equal to the first voltage.
 8. The dischargelamp control device as defined in claim 2, the discharge lamp drivecontrol section including a constant current control correction sectionwhich causes the discharge lamp current when the discharge lamp voltageis lower than a third voltage which is equal to or lower than the firstvoltage to be lower than the discharge lamp current when the dischargelamp voltage is equal to the first voltage.
 9. The discharge lampcontrol device as defined in claim 3, the discharge lamp drive controlsection including a constant current control correction section whichcauses the discharge lamp current when the discharge lamp voltage islower than a third voltage which is equal to or lower than the firstvoltage to be lower than the discharge lamp current when the dischargelamp voltage is equal to the first voltage.
 10. The discharge lampcontrol device as defined in claim 4, the discharge lamp drive controlsection including a constant current control correction section whichcauses the discharge lamp current when the discharge lamp voltage islower than a third voltage which is equal to or lower than the firstvoltage to be lower than the discharge lamp current when the dischargelamp voltage is equal to the first voltage.
 11. The discharge lampcontrol device as defined in claim 5, the discharge lamp drive controlsection including a constant current control correction section whichcauses the discharge lamp current when the discharge lamp voltage islower than a third voltage which is equal to or lower than the firstvoltage to be lower than the discharge lamp current when the dischargelamp voltage is equal to the first voltage.
 12. The discharge lampcontrol device as defined in claim 7, when the discharge lamp voltage iswithin a predetermined range lower than the third voltage, the constantcurrent control correction section increasing the discharge lamp currentaccording to an increase in the discharge lamp voltage so that thedischarge lamp current almost successively changes when the dischargelamp voltage is in the vicinity of the third voltage.
 13. The dischargelamp control device as defined in claim 8, when the discharge lampvoltage is within a predetermined range lower than the third voltage,the constant current control correction section increasing the dischargelamp current according to an increase in the discharge lamp voltage sothat the discharge lamp current almost successively changes when thedischarge lamp voltage is in the vicinity of the third voltage.
 14. Thedischarge lamp control device as defined in claim 10, when the dischargelamp voltage is within a predetermined range lower than the thirdvoltage, the constant current control correction section increasing thedischarge lamp current according to an increase in the discharge lampvoltage so that the discharge lamp current almost successively changeswhen the discharge lamp voltage is in the vicinity of the third voltage.15. The discharge lamp control device as defined in claim 7, the firstvoltage being equal to the third voltage.
 16. The discharge lamp controldevice as defined in claim 8, the first voltage being equal to the thirdvoltage.
 17. The discharge lamp control device as defined in claim 10,the first voltage being equal to the third voltage.
 18. The dischargelamp control device as defined in claim 12, the first voltage beingequal to the third voltage.
 19. The discharge lamp control device asdefined in claim 1, the discharge lamp being a light source of aprojector.
 20. A projector comprising: the discharge lamp control deviceas defined in claim 19; the discharge lamp; an image signal inputsection which inputs an image signal; and an image signal output sectionwhich outputs the image signal.